Civil Engineering Materials
Civil Engineering Materials CEE 3030
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Date Created: 10/21/15
CEE 3030 Civil Engineering Materials Introduction to Portland Cement What is cement heps in the hardening process of concrete is a dry highly hydroscopic materialwhen mixed with aggregate and water makes a very strong material only in compression acts as a glue and holds aggregate together powder form There are many different types of cements undergoes a chemical reaction when mixed with water made with limestone bought in bags What is concrete a mixture of water cement sand and rock high compressive strength durable reliable and cheap is porous can be poured into various shapes Society s perception concrete is only grey in color and rough in texture most people do not know the differences between cement and concrete pretty low tech Why study cementbased materials 1 One would not think of using wood for a dam steel for a pavement or asphalt for a building frame but concrete is used for each of these and for many other uses than other construction materials It is used to support to enclose to surface and to fill More people need to know more about concrete than about other specialized materials JW Kelly former ACI President Why study cementbased materials 2 Concrete is the most widely used building material with more than 1 billion tons of cement consumed annually Projection ofAnnual Regional Cement Consumption million tons 1994 2000 2005 Europe 313 393 432 Asia 680 853 1000 Middle East 65 79 82 Africa 63 71 77 North America 90 92 92 South amp Central America 92 118 142 Misc 7 9 10 Total 1310 1625 1835 World Cement v27 No5 May 1996 Why study cementbased materials 3 TABLE 12 Quantities of Materials Used in US Construction 2000 Vollune Weight Material 10 1111 10quot ftquot 10 metric tons 10quot tons 107 3780 9700 640 700 1180 105 1 16 60 15 Timber Concrete Cement Steel Brick and clay products Building stone Nonferrous metals Asphalt Increasing infrastructure demands Increasing population Construction in more aggressive environments Increasing performance expectations 1960 1975 1987 1999 Why is concrete such a popular building material Readily available Relatively inexpensive 7580lyd3 fc3000 psi Possesses excellent resistance to water Ease of placement in a variety of shapes and sizes Advantages Can be cast in variety of shapes Economical Can be produced locally or even on site Relatively easy to tailor Durable Poor conductor of heat and sound Less energy intensive to produce than other construction materials Can be consideredmade green Can be made aesthetically pleasing Concrete can be made aesthetically pleasing maiiliuivllllll r mm ma 39 I m r L lllllllllllllm life 5quot I ll Disadvantages Volume changes Low tensile strength Low ductility Low strengthtoweight ratio The Future of Concrete 1 Despite major advances over the past 35 years in understanding the structure and properties of concrete perceptions about concrete have changed little Generally it is regarded by the public and structural engineers alike as a low tech low performance material But since WE know this is not so why does this perception persist The Future of Concrete 2 Why does this perception persist Slow implementation of innovation Building codes don t encourage use of innovative materials Structural engineers are too conservative Inadequate materials literacy among engineers and contractors No requirements for continuing education Lack of quality control on site Inadequate marketing of concrete CEE 3030 Civil Engineering Materials Chemical Admixtures What are chemical admixtures Admixtures A material other than water aggregates hydraulic cement and fiber reinforcement used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing ACI 116R Terminology Liquid Water MRWR S Reducers Hydration Controi i i 39i HRWRS Anti Freeze 1960 1970 1980 1990 2000 Why Use Admixtures To modify hardened concrete properties Improve resistance to cyclic freezing amp thawing Improve impact and abrasion resistance Inhibit corrosion of embedded metals Inhibit expansion due to alkalisilica reaction ASR Reduce plastic shrinkage cracking Reduce long term drying shrinkage Reduce permeability Produce colored concrete Produce cellular concrete How are they added to concrete 1 Interground with cement 2 Added during mixing usually to mix water 3 Added to concrete after mixing Always added in amounts lt1 by weight of cement a Can be a lifesaver when used right or a disaster when used incorrectly AirEntraining Admixtures Air Entraining Admixtures AEA Table 86 Recommended Total Target Alr Content for Concrete Momma maxlmum Air content percent size aggregate Severe Moderate Mlld mm In exposurequot exposuret exposuret r lt95 36 9 7 5 95 7 6 4 125 X 7 5 4 190 3A 6 5 3 250 1 6 4 3 w m 375 1 15 5 4 2 50 2 5 4 2 75 3 4 3 1 39 Project specifications often allow the air content of the concrete to be within 1 to 2 percentage points of the table target values 39 Concrete exposed to wetfreezethaw conditions deicers or other aggressive agents 1 Concrete exposed to freezing but not continually moist and not in 41 3 contact with deicers or aggressive chemicals 39 quot 39 T Concrete not exposed to freezing conditions deicers or aggressive quot 4 L agents o a 1 39 39lheseair contents apply to the total mix as for the preceding aggre 73 gate Sizes When testing these concretes however aggregate larger than 375 mm 1 in is removed by handpicking or sieving and air content is determined on the minus 375 mm 1 in fraction of mix Tolerance on air content as delivered applies to this value t g i g t Savor Rf manna 1 NegligibldE quotI therinq lgim Air Entraining Admixtures AEA Provide resistance to damage by freezethaw cycles O o v H O 4 0 cs LT1 E 1 395 E 5 Q Air Content Compressive sirenglhpsi 6000 eg Grace Daravair Darex AEA MB AE90 Walercemenl raiio Fig 71 Typical trial mixture or field data strength curves WaterReducing Admixtures Water Reducing Admixtures WRAs 3 1 Molecule Ilth Anionic Polo Group n nu Hydrocarbon chain Without water reducers cement particles tend to flocculate together in low wlc mixtures WRA molecules adsorb along the cement particle surface and attract layers of water preventing flocculation enhancing flow and lubricating the mixture Before Water tied up in flocs unable to lubricate cement paste Dispersant pasficizer Fv h 7 3 Micrograph of cement grains suspended in water Water Reducing Admixtures WRAs WRAs can be used 2 ways 1 2 Higher dosages or overdosage may retard cement hydration Water Reducing Admixture Classification Water I Increased Reduction or Workability Conventional water reducing admixtures WRA Mid range water reducing admixtures High range water reducing admixtures H RWR or Superplasticizers High Range Water Reducing Admixtures HRWRAs Also called superplasticizers 34x more effective than WRAs can reduce water content by 20 30 while maintaining workability 1 Napthalenebased most common less expensive eg Grace Daracem 100 MB Glenium 3000 2 Melaminebased also common can be used in larger dosages 3 Lignosulfatebased 4 Polyacmlates newest most effective most expensive eg Grace Adva 370 MB Glenium 3400 Set Controlling Admixtures Set Controlling Admixtures Set Accelerator speeds up early hydration cold weather concreting higher early strength some contain chloride salts eg CaClZ eg Grace Daraccel MB Pozzolith 122HE eg nonchloride Grace Daraset 200 MB Pozzutec 20 Set Retarders slows down early hydration hot weather concreting avoid cold joints in large pours does not eliminate slump loss use a specialty hydration control admixture eg MB Delvo Pozzolith 100 Grace Daratard 17 Specialty Admixtures Corrosion Inhibitor Can improve passivation of steel andor limit migration of chlorine and moisture through the concrete calcium nitrite amines fatty acids esters eg Grace DCI MB Rheocrete Reduces the surface tension of water in the paste decreasing the stress on the pore walls and reducing shrinkage strain eg MB Tetraguard A820 Grace Eclipse x ll i 1 1 l l l ix ll x l um I u m Used to reduce expansion caused by ongoing alkalisilica reaction eg MB ASRx 30 LN Ion in selfcompact reduce segregat can be used I ing cancrete 866 mg selfconsol39idat MB Rheomac 9 e CEE 3030 Civil Engineering Materials Hardened Cement Paste Microstructure Cement Hydration Hydration chemical combination of cement and water Two primary mechanisms Through solution involves dissolution of anhydrous compounds to their ionic constituents formation of hydrates in solution and eventual precipitation due to their low solubility Topochemical or solidstate hydration reactions take place directly at the surface of the anhydrous cement compounds without going into solution what happens When water is added to cement Dissolution of cement grains Growing ionic concentration in water now a solution Formation of compounds in solution compounds precipitate out as I After reaching a saturation concentration solids hydration products In later stages products form on or very near the surface of the anhydrous cement e 2 A IN 141 v awn r Cement Hydration 10pm LocoM a Unhydrated Section of poly rnlneralic grain scale of interstitial phase is sligle exaggeratedt Cement Hydration b 10min c Some C3A Nor Fsst reacts wtth calcium sulphate in solution Amorphousalurninate rich get forms on the surface and short AFt rods nucleate at edge of gel and in solution Reaction of C38 to produce 39outer39 product C S H onAFt rad net work leaving Jm between grain surface and hydrated shell 18h e 13days f 1Ldays Secondary Motion CgA reacts with any Sufficient 39inner39 0544 at CaAlSJor F55 produc AFt inside shell forming has formed to ll in the ng long ro at AFt hexagonal plates of Mmspace between gain and C S H inner39 product Continuing formation at shell The 39outer GS H starts to form on irside inner39 product reduces has become more brous of shell from continuing separation of ant rydrom hydration of C35 grain and hydrated shell Fig 73 Development of microstruc39nn e during the hydration of Parrand cement From S54 Development of Microstructure Anhydrous cement Water Development of Microstructure CS H Ettringite CH Development of Microstructure CSH Ettringite CH Development of Microstructure CSH Ettringite CH Development of Microstructure CSH Ettringite CH Development of Microstructure CSH CH Monosulfate 75 quot 39 I V 39 A 9 39 7 rquot Mquot 39 W r rgrv x yr yv L39v 1 H r x L 7 39 U l rquot i quot39 3 71 f39 7 L J 7 v 7 1 H 39 W39s 139 x L 7 J Hydrated Cement Paste Solids Voids CSH Entrapped airgt1mm CH Entrained air 75500um Ettringite Capillary pores macro gt Monosulfate meso Residual unhydrated cement 39quottquot39ayequot space micropores Water Capillary water Adsorbed water lnterlayer water Chemically combined water air void Hexagonal O O O CMOH2 or low sulfate 7 Ebrrcined air bubbles in pasta 3977 39 X spacing between 39 MaxIspumn g of C S H sheets l enfmuned mr for g j CID VOW durablllry m Aggregation of C 3 H particles UKC OlIUm 00lpm 0l um lpm 10pm lOU um lmm TC39mm lnm 10 nm 13900 nm lOOO nm 104 nm 105 nm 106 mm 10quot nm Figure 2 7 Dimensional range of solids and pores in a hydrated cement paste Type 5mm size shapear1d distribution of phases present macrostructu re can be seen unaided 200 um or larger microstructure must been observed with the aid of a microscope Structure of Concrete Each of the phases may be heterogeneous in its composition both solids and voids Relative proportions and characteristics of the phases vary with mixture composition time environment etc All of these factors make predictions of concrete behavior more challenging than predictions for other materials Microstructu re In solids microstructural inhomogeneities can lead to serious effects on strength and other related mechanical properties because these properties are controlled by the microstructural extremes not by the average microstructure Thus the presence of voids cracks and other defects play an important role in determining the performance of the composite material Why do these defects exist in concrete Why do these defects exist in concrete Some voids result from the intrinsic nature of the cement hydration process Other voids are introduced intentionally or unintentionally during mixing andor placing Microcracks and cracks can develop due to mismatch between the components ie different CTE E Microcracks and cracks can develop due to loading and environment In addition to the coarse aggregate fine aggregate and paste together the mortar fraction an important 3rel phase generally exists the transition zone T2 or interfacial transition zone ITZ 39 the interfacial region between the coarse aggregate and the hcp 1 1050 um thick 39 the weakest link The ITZ is the region 1050um wide around coarse aggregate characterized by I u grcgulr 4t r 39 rugb 4 F dr 7 7 11711191110 MW Bulk cement pamlu39 ITZ Because of the increased volume available for crystal growth calcium hydroxide crystals formed in this region tend to be larger and form oriented layers serving as preferred sites for cleavage In addition microcracks tend to form in the transition zone even before the concrete is loaded due to differential shrinkage and drying Amount of microcracking related to aggregate size gradation cement content wc degree of consolidation curing conditions RH thermal history Stressstrain behavior for both the aggregate and cement paste alone are nearly linear elastic But because of the lTZ concrete displays some nonlinear and inelastic behavior in compression ggregare Concrete Cement Pnsre Slruw l IT By tailoring the concrete mixture to reduce 39 the influence ofthe ITZ strength E and L A if 39 m A a impermeability are increased 139 39 vft mx r r Lower wlc A f Higher cement content SCMs Smaller MSA Reactive dolomiti c aggregate Lightweight aggregate T Extended moist curing VoidsPorosity Simplified Power s Model Powers developed a simple empirical model to estimate the amount of capillary porosity in a cement paste with varying degrees of hydration and at different waterto cement ratios Assumptions 1 cm3 of cement produces 2 cm3 hydration products on full hydration d 10 Constant volume in system Simplified Power s Model Case A Increasing Degree of Hydration Consider a paste with We of 063 What is the capillary porosity at 7 days assuming the cement is 50 hydrated 28 days 75 hydrated 365 days 100 hydrated Simplified Power s Model Case B Increasing wlc Assuming 100 hydration what is the capillary porosity for wc070 060 050 and 040 VoidsPorosity Voids are 50 100 Hydration intrinsically part of 40 the hydrated cement paste due to the excess water used for economy and workability in the vast majority of mixtures 10 30 20 Capillary Porosity I I I I I 030 040 050 060 070 080 090 WICM VoidsPorosity 100 The presence of VOIdS affeCtS g Degree of hydration 60 C 2 E i 50 g I 1 5 50 3 3 40 U 5 Ca illary porosity g a O D 139 30 2 E 0 I I I I I I 20 0 20 20 30 40 50 60 Curing time days Compressive strength MPa 100 50 Strength Permeability 10 20 30 40 50 Capillary porosity quot0 vol 100 50 Permeability coefficient X 1039 H1115 Voidsl Porosity Inverse relationship between strength fc and porosity p fc k1p3 k strength of voidless mortar 34000 psi ermeability 9E porosity